winsys/svga: remove useless assertion
winsys/svga: remove useless assertion

An unsigned int is always >= 0.  Found with Coverity.

Reviewed-by: Charmaine Lee <67ce69700661ea856134ddb10419a330e5f037d7@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler